Method of calibrating a write current-setting for servo writing a disk drive

ABSTRACT

A method of calibrating a write current-setting for writing servo sectors onto a recording surface through a head in a head disk assembly of a disk drive is disclosed. A preamplifier circuit has an input for receiving a selected control signal set by a current-setting value, the preamplifier circuit causing current to flow through the head with a current magnitude determined by the current-setting value. A multiple-pass process is performed in which a series of current-setting values are set for the control signal for generating a plurality of quality metrics each indicative of a quality of the selected control signal. Each pass in the multiple-pass process includes the steps of providing a data sequence to the preamplifier circuit to cause a test pattern to be written to the recording surface, and reading the test pattern and generating and storing at least one of the plurality of quality metrics. The generated and stored quality metrics are then evaluated to select a current-setting value for the selected control signal. The selected current-setting value is then set for the head, and the servo sectors are written onto the recording surface through the head.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to disk drives. More particularly, thepresent invention relates to a method of calibrating a writecurrent-setting for servo writing a disk drive.

2. Description of the Prior Art

Magnetic disk drives for computer systems typically employ an array ofdisks and associated read/write heads together with head positioning andspindle mechanics. This arrangement of heads and fixed disk array isreferred to as a head disk assembly or HDA, an overview of which isprovided in FIG. 1A. Several magnetic disks 2 connected in an array arerotated by a spindle motor. Each recording surface (top and bottom) ofeach magnetic disk is accessed through a dedicated head 4; as the disksspin, a thin layer air-bearing forms between the heads 4 and therecording surface such that the heads 4 are said to “fly” just above therecording surface. The heads 4 are connected to the distal end ofactuator arms 6 which are connected to a pivot 8 actuated by a rotaryvoice coil motor (VCM). As the VCM rotates the actuator arms 6 about thepivot 8, the heads 4 are positioned radially over the recording surfaceso that information can be written to and read from the recordingsurface.

The recording surface of the magnetic disk is coated with a thin filmmedium (e.g., cobalt alloy) which is magnetized inductively by a writecoil of the head 4. The digital data being recorded modulates a currentpassing through the write coil in order to inductively write a series ofmagnetic transitions onto the disk surface (recording surface) of thedisk, where a preamplifier chip incorporated within the HDA performs themodulation function in response to the digital data. As shown in FIG.1B, the data is written in the radially spaced, concentric tracks 10which are partitioned into blocks of data referred to as data sectors12. Because the circumferential recording area increases from the innerto outer diameter tracks, more data can be stored in the outer diametertracks. Thus, in order to maintain a more constant linear bit densityand thereby maximize the overall storage capacity, the recording surfaceis normally partitioned into a number of zones where each zone comprisesa predetermined number of tracks. Data is then written to the recordingsurface at an increasing rate as the head traverses radially from theinner to outer diameter zones, thereby increasing the amount of datastored in the outer diameter tracks. This is illustrated in FIG. 1Bwhich shows a disk partitioned into an inner diameter zone 14 comprisingseven data sectors per track, and an outer diameter zone 16 comprisingfourteen data sectors per track. In practice, the recording surface isactually partitioned into several zones with the data rate incrementallyincreasing from the inner to outer diameter zones in order to exploitthe maximum storage capacity of the recording surface.

Typically the magnetic disks 2 also comprise embedded servo sectors 18which are recorded at a regular interval and interleaved with the datasectors 12 as shown in FIG. 1B. A servo sector, as shown in FIG. 1C,typically comprises a preamble 20 and sync mark 22 for synchronizing tothe servo sector; a servo data field 24 comprising coarse positioninformation, such as a Gray coded track address, used to determine theradial location of the head with respect to the plurality of tracks; anda plurality of servo bursts 26 recorded at precise intervals and offsetsfrom the track centerlines which provide fine head position information.When writing or reading data, a servo controller performs a “seek”operation to position the head over a desired track; as the headtraverses radially over the recording surface, the Gray coded trackaddresses in the servo data field 24 provide coarse position informationfor the head with respect to the current and target track. When the head4 reaches the target track, the servo controller performs a trackingoperation wherein the servo bursts 26 provide fine position informationused to maintain the head over the centerline of the track as thedigital data is being written to or read from the recording surface.

The servo sectors 18 are written to the recording surfaces as part ofthe manufacturing process to enable the seek and tracking operationsnecessary to write and read the data sectors 12. A common mechanism forwriting the servo sectors to the recording surfaces is an external servotrack writer which uses the write preamplifier electronics and headswithin the HDA, but which uses separate control circuitry and servomechanics for radially positioning the heads using well known techniquessuch as a laser interferometer. A significant cost reduction can beachieved by a “self-servowriting” method which can use circuitry in thedisk drive for writing the servo sectors.

It is desirable to expedite the process of writing the servo sectors 18to the array of recording surfaces within each disk drive to maximizemanufacturing throughput. It is known to write the servo sectors 18 toall of the recording surfaces simultaneously by using a techniquereferred to as “bank servo writing” wherein the write current generatedby the preamplifier is applied to all of the heads to simultaneouslywrite the servo sectors to all of the recording surfaces rather than onesurface at a time. This is illustrated by the prior art preamplifiershown in FIG. 4 wherein a register 28 is loaded with a digital writecurrent setting converted into an analog write current setting 30 by adigital-to-analog converter (DAC) 32. The analog write current setting30 adjusts the output current of driver circuits (34 ₀-34 _(N)) whichsupply the respective write currents (36 ₀-36 _(N)) to the heads 4. Headselect circuitry 38 within the preamplifier enables the output of theappropriate driver circuit (34 ₀-34 _(N)) over line 40 during normaloperation of the disk drive, and it enables the output of all the drivercircuits (34 ₀-34 _(N)) during servo track writing in order to write theservo sectors to all of the recording surfaces simultaneously. Thedigital write data 42 to be recorded to the surface of the disk 2modulates the operation of the driver circuits (34 ₀ 34 _(N)) byalternating the polarity of the write current 36; for example, a digital“1” bit may modulate a positive write current and a digital “0” maymodulate a negative write current.

Noise in the disk drive (electronic noise, media noise, intersymbolinterference, etc.) may induce errors when reading the track addressesand/or servo bursts which will degrade the performance of the disk driveby increasing seek times as well as increasing the bit error rate if thehead is unable to maintain proper centerline tracking. Therefore, whenthe servo sectors are written to the recording surfaces, it is importantthat enough write current is supplied to each head to saturate themagnetic material on the recording surface so as to maximize the signalpower during read back. Prior art servo track writers that perform abank servo write to all of the recording surfaces simultaneously wouldset the write current high enough to ensure that each head would bedriven by enough current to saturate the recording surfaces. Setting thewrite current higher than the minimum required to saturate the recordingsurface does not significantly reduce the signal-to-noise ratio whenusing a conventional inductive head which comprises a single coil forboth writing and reading the magnetic transitions. This is because thepoles in a conventional inductive head are essentially the same widthwhich results in minimal fringing fields emanating from the periphery ofthe write gap even if the write current is set higher than necessary.This is not the case, however, with magneto-resistive (MR) heads whichcomprise an inductive write element (write coil) and a MR read elementintegrated into one head. In typical MR heads having two poles, one poleof the inductive write element is shared with one of the shields for theMR read element; this pole is consequently wider than the other pole ofthe inductive write element which causes significant fringing fields atthe periphery of the write gap if the write current is set too high.Further, the amount of write current necessary to saturate the recordingsurface varies between the MR heads in the disk array due to processvariations in manufacturing the MR heads and the magnetic disks. Thus,using the prior art preamplifier of FIG. 4 to drive all the MR headswith a single write current high enough to ensure that all of therecording surfaces are saturated may inevitably drive at least one ofthe MR heads with too much write current and cause significant fringingfields.

The fringing fields, if strong enough, will effectively erase an area ofthe disk at the periphery of the write gap thereby forming an “eraseband” along the edges of the servo sector data as well as the servobursts. This is illustrated in FIG. 2A which shows the two write polesof an MR head, where the second write pole is shared with a shield ofthe MR read element and therefore is wider than the first write pole.The view of the MR head in FIG. 2A is looking up from the disk with thedirection of the MR head and orientation of the track vertical to thepage. In addition to the flux lines generated in the write gap betweenthe two poles of the inductive write coil, flinging fields are generatedat the periphery of the write gap due to the disparate pole widths. Asillustrated, the fringing fields arc from the write pole forming fluxlines perpendicular to the track which can effectively erase therecording surface. The width of the adverse fringing fields extends tothe critical flux line, the flux line strong enough to change themagnetization of the recording surface, which is proportional to thestrength of the write current. In FIG. 2A, the write current is too highcausing wide erase bands at the edges of the track. A more optimal writecurrent is shown in FIG. 2B which is just strong enough to generate fluxin the write gap to saturate the recording surface along the track,while creating only a narrow erase band due to the attenuated fringingfields.

The magnetic transitions in the servo track addresses are recorded usinga phase coherent Gray code meaning that the magnetic transitions in thetrack addresses of adjacent tracks differ by only two adjacent bit cellsso that there is no intertrack interference when the head is betweentracks during a seek operation. An erase band caused by the fringingfields of an MR head interferes with the accurate detection of the trackaddresses by disrupting the phase coherent nature of the Gray code. Inaddition, the erase band at the edges of the servo bursts introduces anon-linear distortion in the position error signal generated duringtracking which offsets the centerline position of the head preventingoptimal detection of the data sectors.

Furthermore, the characteristics of the storage medium may change fromthe inner diameter tracks to the outer diameter tracks such that more orless write current may be necessary to saturate the recording surfacedepending on the head's radial location. In addition, since the headtraverses in an arc trajectory, the characteristics of the erase bandformed by the fringing field may vary depending on the radial locationof the head. As the head traverses radially over the disk, the poles ofthe write element will skew from the track centerline depending on thehead's arc trajectory, which changes the characteristic of the eraseband.

There is, therefore, the need to determine the optimal write currentwhile servo track writing a magnetic disk to ensure that the recordingsurface is saturated while avoiding erase bands caused by fringingfields when the write current is set too high. Further, there is a needto determine the optimal write current for a plurality of heads used tosimultaneously write the servo sectors to a plurality of recordingsurfaces in a disk array (bank servo write) so as to maximize themanufacturing throughput. In addition, there is a need to optimize thewrite current with respect to the radial location of the head tocompensate for the varying characteristics of the magnetic media as wellas the varying characteristics of the erase bands as the head skews fromthe track centerline.

SUMMARY OF THE INVENTION

The invention can be regarded as a method of calibrating a writecurrent-setting for writing servo sectors onto a recording surfacethrough a head in a head disk assembly of a disk drive. A preamplifiercircuit has an input for receiving a selected control signal set by acurrent-setting value, the preamplifier circuit causing current to flowthrough the head with a current magnitude determined by thecurrent-setting value. A multiple-pass process is performed in which aseries of current-setting values are set for the control signal forgenerating a plurality of quality metrics each indicative of a qualityof the selected control signal. Each pass in the multiple-pass processincludes the steps of providing a data sequence to the preamplifiercircuit to cause a test pattern to be written to the recording surface,and reading the test pattern and generating and storing at least one ofthe plurality of quality metrics. The generated and stored qualitymetrics are then evaluated to select a current-setting value for theselected control signal. The selected current-setting value is then setfor the head, and the servo sectors are written onto the recordingsurface through the head.

The invention can also be regarded as a method of calibrating aplurality of write current-settings for writing servo sectors onto aplurality of recording surfaces through a plurality of heads in a headdisk assembly of a disk drive. A preamplifier circuit has an input forreceiving a selected control signal set by a current-setting value, thepreamplifier circuit causing current to flow through a selected one ofthe heads with a current magnitude determined by the current-settingvalue. The selected one of the heads is positioned over a respective oneof the recording surfaces. A multiple-pass process is performed in whicha series of current-setting values are set for the control signal forgenerating a plurality of quality metrics each indicative of a qualityof the selected control signal. Each pass in the multiple-pass processincludes the steps of providing a data sequence to the preamplifiercircuit to cause a test pattern to be written to the recording surface,and reading the test pattern and generating and storing at least one ofthe plurality of quality metrics. The generated and stored qualitymetrics are then evaluated to select a current-setting value for theselected control signal. The above steps are then repeated for theremaining heads. The selected current-setting values are then set foreach of the heads, and the servo sectors are simultaneously written ontothe plurality of recording surfaces through the plurality of heads.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a conventional head disk assembly (HDA) within aconventional disk drive comprising an array of disks and associatedheads positioned radially over the disk surfaces.

FIG. 1B shows a typical format for one of the disk surfaces in the diskarray of FIG. 1A comprising a plurality of radially spaced, concentricdata tracks partitioned into a number of data sectors and furthercomprising embedded servo sectors for positioning the heads over thedisk surfaces while seeking and tracking.

FIG. 1C shows a typical format of an embedded servo sector comprising apreamble and sync mark for synchronizing to a servo data fieldcomprising coarse track positioning information such as a track address,and further comprising servo bursts recorded at precise intervals andoffsets with respect to the track's centerline which provide fineposition information during tracking.

FIG. 2A illustrates the geometry of the write poles in amagneto-resistive (MR) head and how the disparity in the width of thewrite poles generates fringing fields at the periphery of the write gapwhich results in wide erase bands at the edges of the servo tracks ifthe write current is set too high.

FIG. 2B illustrates how the erase bands are attenuated in the MR head ofFIG. 2A by optimizng the write current.

FIG. 3A is a block diagram of a disk drive comprising a HDA togetherwith a read/write channel, disk controller and servo controller mountedon a printed circuit board (PCB), wherein the HDA comprises a n-currentpreamplifier for generating independent write currents optimized foreach head.

FIG. 3B shows the HDA of FIG. 3A inserted into an external servo trackwriter during manufacturing which calibrates the optimal write currentsfor each head in the disk array and then simultaneously writes the servosectors to all of the recording surfaces in a bank servo write mode.

FIG. 3C shows a disk drive employing “self-servowriting” to write theservo sectors to all of the recording surfaces during manufacturing bycalibrating the optimal write currents and performing the bank servowrite operation internal to the disk drive.

FIG. 4 illustrates the composition of a conventional preamplifier whichgenerates the same write current for each head while bank servo writingthe disk array.

FIG. 5A is a block diagram of the n-current preamplifier of FIG. 3Acomprising an input for receiving a plurality of current-setting controlsignals, and a plurality of signal-to-current converters for convertingthe current-setting control signals into independent write currents.

FIG. 5B shows one embodiment for the n-current preamplifier of FIG. 5A,wherein a plurality of registers store the current-setting controlsignals, and a plurality of digital-to-analog converters (DACs) andcorresponding driver circuits constitute the signal-to-currentconverters for converting the current-setting control signals intoindependent write currents.

FIG. 5C illustrates an alternative embodiment for the n-currentpreamplifier of FIG. 5A, wherein the independent write currents aregenerated by adding a global (coarse) write current setting generatedfor all of the heads to a local (fine) write current setting for eachhead in order to reduce the circuitry in the DACs as compared to theimplementation of FIG. 5B.

FIG. 5D shows yet another alternative embodiment for the n-currentpreamplifier of FIG. 5A which further reduces the circuitry used togenerate the independent write currents by using a single register-DACpair and a plurality of sample-and-hold (S/H) circuits for sampling theoutput of the DAC, wherein the DAC's register is set with theindependent write current settings just prior to writing a servo sectorto the recording surfaces.

FIG. 6A illustrates an example embodiment for the driver circuitsemployed in the n-current preamplifier of FIGS. 5B-5D.

FIG. 6B illustrates a conventional circuit for generating the same writecurrent for all the heads in the prior art preamplifier of FIG. 4.

FIG. 7A shows an example circuit for generating the independent writecurrents for each driver circuit in the n-current preamplifier of FIG.5B.

FIG. 7B shows an example circuit for generating the independent writecurrents for each driver circuit in the n-current preamplifier of FIG.5C.

FIGS. 8A-8B are flow diagrams illustrating a write current calibrationand bank servo writing procedure that employs the n-current preamplifierof FIG. 5 for calibrating independent write currents and simultaneouslyservo writing a plurality of recording surfaces.

DESCRIPTION OF THE PREFERRED EMBODIMENTS System Overview

An overview of a disk drive 44 employing the aspects of the presentinvention is shown in FIG. 3A. The disk drive 44 is connected to a hostcomputer via interface connection 46. The host computer transmits userdata to be stored to the disk drive, and receives information read fromthe disk drive 44. The user data is stored on the recording surfaces ofan array of magnetic disks 2 rotated by a spindle motor 48 locatedinside an HDA 49. A voice coil motor (VCM) 50 actuates a pivot 8 torotate an array of actuator arms 6 with a plurality of heads 4 attachedto the actuator arms 6 in order to position the heads 4 radially overthe recording surfaces. Heads 4 comprise an inductive write coil forwriting magnetic transitions on the recording surface, and amagneto-resistive (MR) read element for reading the magnetictransitions.

The recording surfaces of the magnetic disks 2 comprise a plurality ofconcentric, radially spaced tracks 10 partitioned into a number of datasectors 12 as illustrated in FIG. 1B. User data is written to therecording surface a data sector at a time; the recording surfacescomprise servo sectors 18 which facilitate positioning the head 4 over adesired data sector within a particular track 10. A servo controller 52includes a track positioning system that processes Gray coded trackaddresses within the servo sectors 18 to position the head 4 withrespect to the tracks 10 during seek operations, and it processes servobursts within the servo sectors 18 to maintain the head over thecenterline of the track (tracking) as data is written to or read fromthe disk 2. The VCM 50 for positioning the heads 4 is preciselycontrolled by the servo controller 52 in response to a position errorformed from the track address and servo burst information. The servocontroller 52 also includes a spindle motor control system that controlsa spindle motor 48 which rotates the array of magnetic disks. Thespindle motor 48 is controlled by the servo controller 52 when the diskdrive is initially powered on in order to “spin-up” the disk array.During normal operation, the servo controller 52 controls the spindlemotor 48 so that the disk array rotates at a substantially constantangular velocity.

A disk controller 54 coordinates the operation of the disk drive byhandling the host interface functions when a host request to write orread data is received, and controlling other components in the diskdrive to effectuate the write and read operations to and from the disks2. A read/write channel 58 comprises encoding circuitry for encoding thedata prior to writing the digital write data 42 to the disk 2, as wellas signal processing circuitry for processing the analog read signal 62when reading data from the disk 2.

A n-current preamplifier 64 (the details for which are described below)generates the write currents 66 _(i) for each head 4 during writeoperations, and it preamplifies the analog read signal 65 _(i) emanatingfrom the head 4 during read operations.

During a write operation, user data to be written to the disk 2 isencoded using an error correction code (ECC), such as the well knownReed-Solomon code, which during read operations is used to detect andcorrect errors in the data due to noise induced by the electronics andother imperfections in the recording/read-back process. The ECC encodingis suitably carried out by the disk controller 54 which then transfersthe ECC encoded data to the read/write channel 58 over line 68. Theread/write channel 58 may further encode the ECC data using a channelcode, such as a run-length limited (RLL) channel code, designed toenhance the performance of the disk drive by placing certain constraintson the recorded data which increases the effective signal-to-noise ratio(SNR) during read back. The encoded write data is transferred over line42 to the n-current preamplifier 64 where it modulates the write current66 _(i) in an inductive write coil of the head 4 in order to write aseries of magnetic transitions on the recording surface of the disk 2which represent the recorded data. For example, a “1” bit may modulate apositive write current and a “0” bit may modulate a negative writecurrent. The disk controller 54 programs the n-current preamplifier 64with a head select signal over line 70 for selecting the appropriatehead 4 when the encoded user data is written to a particular recordingsurface.

During a read operation, the MR element in the head 4 senses themagnetic transitions recorded on the recording surface of disk 2 andgenerates analog read signal 65 _(i) comprising polarity alternatingpulses representing the recorded digital data. The n-currentpreamplifier 64 preamplifies the analog read signal 65 _(i) andtransmits analog read signal 62 to the read/write channel 58. Theread/write channel 58 comprises circuitry for evaluating the pulses inthe analog read signal 62 in order to demodulate the recorded data. Itmay comprise a simple analog peak detector for detecting isolated pulsesin the analog read signal 62, or it may comprise a partialresponse/maximum likelihood (PRML) detector which samples the analogread signal 62 and then estimates the recorded digital data byevaluating the signal samples in context to determine a most likely datasequence associated with the signal samples. The preferred embodiment isto use the PRML read/write channel because it reduces the error rate fora given SNR and thus the overall storage capacity of the disk drive.After detecting an estimated data sequence from the analog read signal62, a channel decoder in the read/write channel 58 decodes the estimateddata sequence which is then transferred to the disk controller 54 overline 68 for ECC decoding. After ECC decoding and integrity verification,the decoded user data is transferred to the host via the disk driveinterface 46.

In FIG. 3A, the read/write channel 58 also detects the data in the servosectors 18 which is transferred to the servo controller 52 over line 76for positioning the heads 4 with respect to the tracks 10 during bothread and write operations (seeking and tracking). During seeks, theservo controller 52 computes a coarse position error as the differencebetween the current track location of the head 4 as specified by thetrack address in the servo data field 24 of the servo sector 18, and atarget track provided by the disk controller 54 over line 78. When thehead 4 reaches the target track, the servo controller 52 computes aposition error signal (PES) with respect to the track centerline fromthe position information provided by the servo bursts 26; the servocontroller 52 makes fine adjustments to the VCM 50 in order to drive thePES to zero and thereby maintain centerline tracking while reading datafrom or writing data to the disk 2.

The n-current preamplifier 64 is typically implemented as a separateintegrated chip located inside the HDA 49 near the actuator arms 6 ofthe actuator assembly in order to minimize noise interjected into theread/write signal over lines 65 and 66 which connect the n-currentpreamplifier 64 to the heads 4. The remaining components shown in FIG.3A, the read/write channel 58, disk controller 54, and servo controller52 may be implemented as separate chips or as a single integrated chiptypically mounted on printed circuit board (PCB) 80 attached to thebottom of the HDA 49 and connected to the n-current preamplifier 64through a cable conductor. To minimize the number of pins and associatedcost of the n-current preamplifier 64, the interface between then-current preamplifier 64 and other circuitry located on the PCB(read/write channel 58 and disk controller 54) is preferably implementedusing a serial interface.

During the manufacture of the disk drive 44 shown in FIG. 3A, either anexternal servo track writer or self-servowriting is employed tosimultaneously write the servo sectors 18 to all of the recordingsurfaces using the n-current preamplifier 64. FIG. 3B shows the HDA 49of FIG. 3A inserted into an external servo track writer 77 before thePCB 80 has been mounted to the HDA 49. The external servo track writer77 comprises a “clock head” 79 positioned over one of the recordingsurfaces and a clock pattern generator 81 for writing a magnetic clockpattern in a track preferably at an outer diameter of the recordingsurface. When bank writing the servo sectors 18, the magnetic clockpattern is read by the clock head 79 and processed by a timing circuit83 which generates a timing clock 85 applied to a controller 87. Thecontroller 87 preferably processes the timing clock 85 to derive theprecise circumferential location of the heads 4 with respect to thetracks so that the servo sectors 18 are written at the samecircumferential location from the inner to outer diameter tracks. Othersuitable methods are known in the art for generating the timing clock85, including an optical clock pattern recorded on an inner diameter ofa recording surface which is read using an optical transducer comprisinga light source and a photodetector.

The external servo track writer 77 further comprises a push pin 89 whichis inserted into the HDA 49 and into a hole in the actuator arm 6. Ahead positioner circuit 91, suitably comprising a very fine resolutionstepper motor, actuates the push pin 89 in order to precisely positionthe heads 4 radially over the disk 2 while writing the servo sectors(servo data and servo bursts). The controller 87 applies a reversedirection bias current to the coil of the VCM 50 over line 93 in orderto bias the actuator arm 6 against the push pin 89 to facilitate precisepositioning of the heads 4. The controller 87 also applies a current tothe coil of the spindle motor 48 over line 95 in order to spin up thedisks 2 and then rotate the disks 2 at a substantially constant angularvelocity.

Before writing the servo sectors 18 to the recording surfaces of thedisks 2, the controller 87 executes a calibration procedure (the detailsof which are set forth below with respect to FIGS. 8A and 8B) in orderto determine optimal write current-setting control signals for theplurality of heads 4. The calibration procedure executed by thecontroller 87 involves programming the n-current preamplifier 64 overline 70 with a write current-setting control signal for a particularhead 4, writing a test pattern (write data 42) to the respectiverecording surface, reading the test pattern, and processing the readsignal 62 to generate a quality metric indicative of the quality of thewrite current setting. Several write current-setting control signals aretested for each head 4, and the write current-setting control signalsthat generate the best (optimal) quality metric are used during the bankservo write operation. The external servo track writer 77 comprises aquality metric measurement circuit 97 for processing the read signal 62to generate the quality metric 99 evaluated by the controller 87 duringthe calibration process. The quality metric measurement circuit 97preferably computes an overwrite measurement which provides anindication of how well the write current saturates the recordingsurface. As explained in greater detail below, the overwrite measurementinvolves writing a low frequency test pattern to the recording surfaceand measuring the energy in the read signal at the low frequency uponread back, overwriting the low frequency test pattern with a highfrequency test pattern, measuring the residual energy in the lowfrequency component of the read signal upon read back, and taking theratio of the first low frequency energy measurement to the residual lowfrequency measurement (after the overwrite) to generate the overwritemeasurement. The quality metric measurement circuit 97 (in analog ordiscrete-time) filters the read signal 62 and measures the energy in theread signal 62 to generate the overwrite quality metric 99. Once theoptimal write current-setting control signals have been determined foreach head 4 in the disk array, the controller 87 programs the n-currentpreamplifier 64 over line 70 with the optimal write current-settingcontrol signals and then performs the bank servo write operation bysimultaneously writing the servo sectors 18 to all of the recordingsurfaces using the precise timing clock 85. The servo sector data ispassed from the controller 87 over write data line 42, through then-current preamplifier 64, and over lines 66 connecting the n-currentpreamplifier 64 to the write coil of the heads 4.

An alternative method for writing the servo sectors to the recordingsurfaces of the disks 2 during manufacturing is to utilize the diskcontroller 54, read/write channel 58, and servo controller 52 alreadyintegrated into the disk drive 44 of FIG. 3A to “self-servowrite” theservo sectors 18. This technique suitably entails an iterative processwherein each servo track is written using information from a previouslywritten servo track. As shown in FIG. 3C, the disk controller 54executes the calibration procedure and bank servo write operationdescribed below. During the calibration procedure the disk controller 54programs the n-current preamplifier 64 over line 70 to select theappropriate head 4 and to set the appropriate write current-settingcontrol signal, and the quality metric 74 used to calibrate the optimalwrite current setting is generated by the read/write channel 58. Whenbank servo writing the servo sectors 18, the disk controller 54 programsthe n-current preamplifier 64 over line 70 with the optimal writecurrent-setting control signals for all of the heads 4 and thensimultaneously writes the servo sectors 18 to all of the recordingsurfaces. The servo sector data is transferred over line 42 from thedisk controller 54 through the read/write channel 58 (unmodified) to then-current preamplifier 64 and over lines 66 connecting the n-currentpreamplifier 64 to the write coil of the heads 4.

N-Current Preamplifier

With reference to FIG. 5A, n-current preamplifier 64 comprises an input103 for receiving a plurality of current-setting control signals overline 70, and a plurality of signal-to-current converters (89 ₀-89 _(N))for converting the current-setting control signals into a plurality ofwrite currents (66 ₀-66 _(N)) for the heads 4, wherein each writecurrent (66 ₀-66 _(N)) has a magnitude that is independently controlledby a respective one of the current-setting control signals. A headselect control signal is also received over line 70 and applied to ahead select circuit 38 which selects all of the heads 4 when bank servowriting the servo sectors to all of the recording surfaces, and selectsthe appropriate head 4 when writing a user data sector to a particularrecording surface during normal operation. The write data received overline 42 to be written to the recording surface are input into thesignal-to-current converters (89 ₀-89 _(N)) to modulate the writecurrents (66 ₀-66 _(N)); for example, a “1” bit may modulate a positivewrite current and a “0” bit a negative write current in the coil of thehead 4. In the embodiment shown in FIG. 5A, the current-setting controlsignals received over line 70 are stored in respective registers 102 andthen converted into the appropriate write currents (66 ₀-66 _(N)) by thesignal-to-current converters (89 ₀-89 _(N)). Several example embodimentsof the signal-to-current converters (89 ₀-89 _(N)) are illustrated inFIGS. 5B-5C.

In the embodiment of the n-current preamplifier 64 shown in FIG. 5B, aplurality of DACs (90 ₀-90 _(N)) driven by a common current reference 92output by current reference generator 94 and a plurality of drivercircuits (100 ₀-100 _(N)) constitute the signal-to-current converters(89 ₀-89 _(N)) of FIG. 5A. Each of the DACs (90 ₀-90 _(N)) generates anindependent analog write current setting (96 ₀-96 _(N)) forindependently setting the write current (66 ₀-66 _(N)) for each head 4through the plurality of driver circuits (100 ₀-100 _(N)). The DACs (90₀-90 _(N)) convert the current-setting control signals stored inregisters 102 into the analog write currents settings (96 ₀-96 _(N)). Asdescribed below with respect to FIGS. 7A and 7B, the driver circuits(100 ₀-100 _(N)) of FIG. 5B are modified to output independent writecurrents (66 ₀-66 _(N)) corresponding to the analog write currentsettings (96 ₀-96 _(N)) for each head 4 rather than output the samewrite current for all the heads as in the prior art preamplifier of FIG.4. When writing data to target data sector on a recording surface duringnormal operation, the head select circuit 38 enables the output of theappropriate driver circuit (100 ₀-100 _(N)) corresponding to therecording surface of the target data sector. When bank writing the servosectors 18, the head select circuit 38 enables the outputs of all thedriver circuits (100 ₀-100 _(N)) so that the servo sectors 18 aresimultaneously written to all of the recording surfaces in the bankservo write mode. The digital write data 42 to be recorded to therecording surface modulates the operation of the driver circuits (100₀-100 _(N)) by alternating the polarity of the write currents (66 ₀-66_(N)). As explained in greater detail below with respect to FIG. 6A, thedigital write data 42 is preferably implemented as a differential signaland the driver circuits (100 ₀-100 _(N)) implemented as differentialamplifiers.

It is desirable to implement the n-current preamplifier 64 efficientlyand cost effectively, which means minimizing the number of externalcontrol pins as well as the internal circuitry. The number of externalcontrol pins can be minimized by providing a serial interface to then-current preamplifier 64 both for the digital write data 42 as well asthe control signals such as the current-setting control signals and headselect control signal received over line 70. To minimize the internalcircuitry, the following description provides two alternativeembodiments for the n-current preamplifier 64 which reduce the circuitryassociated with implementing the DACs (90 ₀-90 _(N)) of FIG. 5B. TheDACs (90 ₀-90 _(N)) of FIG. 5B require numerous transistors to implementand therefore represent a significant portion of the internal circuitryin the n-current preamplifier 64. Because the DACs (90 ₀-90 _(N)) arebinary weighted, the number of transistors required to implement thehigher order bits increases exponentially. However, the optimal writecurrents (66 ₀-66 _(N)) for the individual heads 4 may differ by only asmall amount which means that the higher order bits of thecurrent-setting control signals (stored in registers 102) are all thesame. This characteristic can be exploited to reduce the implementationcost of the DACs (90 ₀-90 _(N)) shown in FIG. 5B while performing thesame function.

One embodiment of the n-current preamplifier 64 which reduces the DACcircuitry is shown in FIG. 5C wherein the individual DACs (90 ₀-90 _(N))of FIG. 5B have been replaced by a global DAC 104 for generating aglobal write current setting 105 (coarse write current setting), and aplurality of local DACs (106 ₀-106 _(N)) for generating a plurality oflocal write current settings (108 ₀-108 _(N)) (fine write currentsettings). Register 110 provides the global current-setting controlsignal to the global DAC 104, and registers 112 provide the localcurrent-setting control signals to the local DACs (106 ₀-106 _(N)). Theglobal write current setting 105 output by the global DAC 104 is addedto the local write current settings (108 ₀-108 _(N)) output by the localDACs (106 ₀-106 _(N)) at adders (114 ₀-114 _(N)) to generate the analogwrite current settings (116 ₀-116 _(N)) for controlling the writecurrents (66 ₀-66 _(N)) output by the driver circuits (100 ₀-100 _(N)).The global DAC 104 and the local DACs (106 ₀-106 _(N)) are all driven bya common current source 92 output by current reference 94 so that theglobal DAC 104 and local DACs (106 ₀-106 _(N)) track together variationsin the current reference 92.

A calibration procedure (described below) is executed to determine theoptimal values for the global current-setting control signal stored inregister 110, as well as the local current setting control signalsstored in registers 112. Suitably, the global current-setting controlsignal could be set to a minimum current-setting control signalcorresponding to the head 4 that requires the least write current in thedisk array, and then set the local current-setting control signalsincrementally higher for the remaining heads. Alternatively, the globalcurrent-setting control signal could be set to a maximum write currentcorresponding to the head 4 that requires the most write current in thedisk array, and then set the local current-setting control signalsincrementally lower for the remaining heads 4. Accordingly, the adders(114 ₀-114 _(N)) of FIG. 5C are designed to either add or subtract thelocal current settings (108 ₀-108 _(N)) from the global current setting105 to generate the analog write current settings (116 ₀-116 _(N)) forcontrolling the driver circuits (100 ₀-100 _(N)).

The accuracy of the write currents (66 ₀-66 _(N)) for each head 4 in thedisk array depends on the resolution and range of the localcurrent-setting control signals, which depends on the number of bitsused to represent the current-setting control signals. Typically, theresolution and range necessary to provide adequate performance is rathersmall so that only a few bits (1, 2 or 3) are needed to represent thelocal current-setting control signals. Because the digital circuitryinternal to the n-current preamplifier 64 (buses, registers, etc.) aretypically 8-bits wide, it would be inefficient to provide a separate8-bit register for each of the local current-setting control signals.Thus, to further reduce the implementation cost of the n-currentpreamplifier 64 of FIG. 5C, registers 112, which are 8-bits wide, storemultiple local current-setting control signals. For example, if two bitswere used to represent each local current-setting control signal, theneach 8-bit register 112 would store four 2-bit local setting controlsignals. For a disk drive comprising four heads 4, the n-currentpreamplifier 64 would require one 8-bit register 110 to store the globalcurrent-setting control signal, and one 8-bit register 112 for storingthe four 2-bit local current-setting control signals.

Another alternative embodiment for the n-current preamplifier 64 of thepresent invention which reduces the circuitry associated withimplementing the DACs (90 ₀-90 _(N)) of FIG. 5B is shown in FIG. 5D.This embodiment employs a single register 118 and DAC 120 configuration,a plurality of sample-and-hold (S/H) circuits (122 ₀-122 _(N)), and aswitch 124 for applying the analog write current setting 126 output bythe DAC 120 to the appropriate S/H circuit 122 _(n). When writing userdata to the disk during normal operation, register 118 is loaded withthe appropriate current-setting control signal corresponding to therecording surface comprising the target data sector. The head selectcircuit 38 sets switch 124 over line 128 to select the corresponding S/Hcircuit 122 _(n) which samples the analog write current setting 126output by the DAC 120 and supplies it to the corresponding drivercircuit 100 _(n), the output of which is also enabled by the head selectcircuit 38 over line 40. When writing the embedded servo sectors 18 tothe disks in the bank servo write mode, the appropriate write current(66 ₀-66 _(N)) for each head 4 is set just prior to writing a servosector 18 by performing the following steps: load the writecurrent-setting control signal into register 118 for each head 4;program the head select circuit 38 to set switch 124 to select theappropriate S/H circuit 122 _(n) to sample and hold the analog writecurrent setting 126 at the output of the DAC 120; and once all of thewrite current settings 126 corresponding to each head have been sampledand are available at the outputs of the S/H circuits (122 ₀-122 _(N)),program the head select circuit 38 to enable the output of all thedriver circuits (100 ₀-100 _(N)) to simultaneously write the servosector 18 to all surfaces of the disks. Alternatively, the n-currentpreamplifier 64 of FIG. 5D could be implemented using a separateregister 118 for each current-setting control signal which would avoidthe latency in transferring the current-setting control signals from thedisk controller 54 to register 118 in the n-current preamplifier 64 overthe serial interface. Further, the n-current preamplifier 64 could bedesigned to continuously resample the analog write current setting 126output by the DAC 120 to continuously refresh the analog write currentsettings (116 ₀-116 _(N)) at the outputs of the S/H circuits (122 ₀-122_(N)) while writing the servo sector 18 to the recording surfaces,thereby compensating for loss in performance due to the S/H circuits(122 ₀-122 _(N)) bleeding.

Those skilled in the art understand how to implement the conventionalcomponents (head select circuit, current source, registers, DAC, S/H,etc.) employed in the n-current preamplifier 64 of the embodiments shownin FIGS. 5B-5D. An example embodiment of the circuitry used to implementthe driver circuits (100 ₀-100 _(N)) of FIGS. 5B-5D is shown in FIG. 6A,and an example embodiment of the circuitry used to generate theindependent write currents (66 ₀-66 _(N)) output by the driver circuits(100 ₀-100 _(N)) is shown in FIG. 7A and FIG. 7B.

The driver circuit 100 _(n) shown in FIG. 6A is a differential amplifierwith the digital write data 42, implemented as a differential signal, asthe differential input. A “1” bit in the digital write data 42 modulatesa positive polarity in the differential input whereas a “0” bitmodulates a negative polarity at the differential input. When thedifferential input is positive (“1” bit), transistors 130 a, 132 a and134 b are turned on while transistors 130 b, 132 b and 134 a are turnedoff causing current to flow from V_(CC) at point B through transistor134 b, through the write coil 136, and through transistor 132 a toV_(EE). When the differential input is negative (“0” bit), transistors130 b, 132 b and 134 a are turned on while transistors 130 a, 132 a and134 b are turned off causing current to flow from V_(CC) at point Athrough transistor 134 a, through the write coil 136, and throughtransistor 132 b to V_(EE). Thus, the polarity of the write currentthrough the write coil 136 is reversed as modulated by the polarity ofthe differential input signal (i.e., the digital write data 42). Themagnitude of the write current 66 _(n) flowing through the write coil136 for each head is controlled by a write current source 138 _(n) ineach driver circuit 100 _(n), where each write current source 138 _(n)is in turn controlled by the analog write current setting 116 _(n)output by the DACs of FIGS. 5B-5D.

A typical configuration for a prior art write current source employed inthe driver circuits (34 ₀-34 _(N)) of the prior art preamplifier of FIG.4 is shown in FIG. 6B. This circuit implements a current mirror; thecurrent 30 generated by the DAC 32 of FIG. 4 and flowing throughtransistor 140 is mirrored in a transistor (142 ₀-142 _(N)) of the writecurrent source in each of the driver circuits (34 ₀-34 _(N)). Whenwriting user data to a recording surface during normal operation, thehead select circuit 38 of FIG. 4 controls the operation of a switch 144in FIG. 6B to connect the base terminal 146 of transistor 140 to thebase terminal 148 _(n) of the appropriate transistor 142 _(n) to enablethe output of the appropriate driver circuit (34 ₀-34 _(N)). Whensimultaneously writing the embedded servo sectors 18 to all surfaces ofthe disk during the bank servo write mode, the head select circuit 38controls the switch 144 to connect the base terminal 146 of transistor140 to all of the base terminals (148 ₀-148 _(N)) of transistors (142₀-142 _(N)) to simultaneously enable the output of all the drivercircuits (34 ₀-34 _(N)). Note that when bank servo writing the servosectors, the write currents (36 ₀-36 _(N)) generated by the drivercircuit transistors (142 ₀-142 _(N)) (in the prior art preamplifier ofFIG. 4) are the same.

FIG. 7A shows an example embodiment for the circuit used to generate theindependent write currents (66 ₀-66 _(N)) output by the driver circuits(100 ₀-100 _(N)) of the n-current preamplifier 64 shown in FIG. 5B. Thiscircuit comprises a separate, independent current mirror for generatingthe write current 66 _(n) in each of the driver circuits (100 ₀-100_(N)) of FIG. 6A. The output of each DAC (90 ₀-90 _(N)) generates acurrent (96 ₀-96 _(N)) in a respective transistor (150 ₀-150 _(N)) whichis then mirrored in a companion transistor (152 ₀-152 _(N)) byconnecting their base terminals as shown. In this manner, the writecurrent for each of the driver circuits (100 ₀-100 _(N)) is setseparately and independently according to the outputs of the DACs (90₀-90 _(N)).

FIG. 7B shows an example embodiment for the circuit used to generate theindependent write currents (66 ₀-66 _(N)) output by the driver circuits(100 ₀-100 _(N)) of the n-current preamplifier 64 shown in FIG. 5C. Thiscircuit comprises a global current mirror for generating the global(coarse) current setting for each of the driver circuits (100 ₀-100_(N)), and a separate local current mirror for generating the local(fine) current settings for each of the driver circuits (100 ₀-100_(N)). The global DAC 104 of FIG. 7B generates a current 105 intransistor 154 which is then mirrored in transistors (156 ₀-156 _(N)) byconnecting their respective base terminals through a switch 158. Whenwriting user data to a recording surface during normal operation, thehead select circuit 38 controls the switch 158 to connect the baseterminal 162 of transistor 154 to the base terminal 160 _(n) of theappropriate write current transistor (156 ₀ -156 _(N)). When bank servowriting the embedded servo sectors to all of the recording surfaces, thehead select circuit 38 controls the switch 158 to connect the baseterminal 162 of transistor 154 to the base terminals (160 ₀-160 _(N)) ofall of the write current transistors (156 ₀-156 _(N)). The local currentsettings for each driver circuit (100 ₀-100 _(N)) is generated by alocal current mirror driven by the output of one of the local DACs (106₀-106 _(N)). For example, the local current setting for the first drivercircuit 100 ₀ is generated by the current mirror formed by local DAC 106₀ generating a current 108 ₀ in transistors 164 ₀ and mirrored intransistor 166 ₀. The current flowing through transistors 166 ₀ and 156₀ are added to generate the write current 66 ₀ for driver circuit 100 ₀.

Write Current Calibration and Bank Servo Writing

A calibration procedure is preferably used for determining theappropriate current-setting control signals for providing the optimalwrite current (66 ₀-66 _(N)) for each head 4 in the disk array beforebank servo writing the servo sectors to the disks 2. The calibrationprocedure can be executed by the controller 87 in the external servotrack writer shown in FIG. 3B. Alternatively, the calibration procedurecan be executed by the disk controller 54 of FIG. 3 in aself-servowriting mode. In any event, the steps of the calibrationprocedure are performed for each head/surface combination, and it mayoptionally be carried out for different areas of each recording surface;for example, the appropriate current-setting control signal may bedetermined for each zone of the recording surface to compensate forvariations in the magnetic characteristics from the inner to outerdiameter tracks. In general, the calibration procedure involvesadjusting the current-setting control signal for a particular head 4,providing the write current (66 ₀-66 _(N)) for writing a test pattern tothe recording surface, reading the test pattern from the recordingsurface, and generating a quality metric indicative of a quality of thecurrent-setting control signal and the associated write current (66 ₀-66_(N)). These steps are iterated for several different current-settingcontrol signals, and the current-setting control signal that generatesthe best (optimal) quality metric is selected as the optimalcurrent-setting control signal for producing the write current to writethe servo sectors.

The quality metric is preferably generated using discrete-time circuitryto facilitate adapting (programming) the calibration procedure to thevarious magnetic disks and heads found in the market. In the self-servowriting embodiment, the read/write channel 58 of FIG. 3C is preferablyimplemented using discrete-time circuitry to implement partialresponse/maximum likelihood (PRML) detection algorithms. A PRMLread/write circuit comprises a channel calibration circuit forcalibrating the optimal write currents for servo writing. Thus, FIG. 3Cshows the read/write channel 58 generating a quality metric 74 that issupplied to the disk controller 54 which performs the calibrationprocedure when self-servowriting the recording surfaces.

With reference to FIG. 8A and FIG. 8B, a write current calibration andbank servo write procedure 800 illustrates calibrating the n-currentpreamplifier 64 of FIG. 5C which comprises a global writecurrent-setting control signal and a local write current-setting controlsignal; however, those skilled in the art are capable of modifying theflow diagrams to conform to the implementation of the n-currentpreamplifiers shown in FIG. 5B and FIG. 5D. In addition, the flowdiagrams illustrate the calibration procedure performed for each zone oneach surface of the disks, but this is not a necessary aspect of theembodiment. It may be sufficient to use a single write current for theentire recording surface, for example, an optimal write current measuredat the center of the disk or an optimal write current computed from anaverage of several write currents measured at different locations on therecording surface (e.g., the average of write currents measured at thevarious zones).

Referring now to FIG. 8A, the first step 170 of the calibrationprocedure 800 is to initialize a variable HEAD, which represents thecurrent head being calibrated, to zero (the first head), and toinitialize a variable ZONE, which represents the current zone on therecording surface, to zero (the first zone). Then at step 172, avariable GLOBAL_CUR, which represents the global current-setting controlsignal stored in register 110 for the global DAC 104 of FIG. 5C, is setto a minimum; a variable LOCAL_CUR, which represents the local currentsetting control signal for a corresponding local current register 112 ofFIG. 5C for the current head being calibrated, is set to a minimum; anda variable QUALITY METRIC, which represents the best quality metricmeasured for the current zone, is set to a minimum. At step 174 the HEADbeing calibrated is positioned over the current ZONE, and at step 176 atest pattern is written to the disk. At step 178 the test pattern isread from the disk, and at step 180 a quality metric is measured inresponse to the test pattern.

One example quality metric that could be generated for use in thecalibration procedure 800 is an “overwrite” measurement. An overwritemeasurement is generated by:

DC erasing a track;

writing a first data sequence to the track;

reading the first data sequence to generate a first read signal having alow frequency signal component that depends on data in the first datasequence;

filtering the first read signal to extract the low frequency signalcomponent, and measuring the energy in the low frequency signalcomponent (energy-lowfreq);

storing in memory the measured energy in the low frequency signalcomponent;

overwriting the first data sequence with a second data sequence;

reading the second data sequence to generate a second read signal; and

filtering the second read signal to extract a residual low frequencysignal component representing the residual of the low frequency signalcomponent, and measuring the energy in the residual low frequency signalcomponent (energy-lowfreq-residual).

The overwrite measurement (quality metric) is then computed as the ratioof the energy in the low frequency signal component to the energy in theresidual low frequency signal component:

Quality Metric=energy-lowfreq/energy-lowfreq-residual.

The overwrite measurement is a good indication of how well the writecurrent (66 ₀-66 _(N)) saturates the recording surface, and therefore itis a good quality metric for use in calibrating the optimal writecurrent (66 ₀-66 _(N)) for writing servo sectors. The optimal writecurrent (and best quality metric) is the minimum write current thatachieves a predetermined overwrite measurement (e.g., 35-40 db).

Continuing now with the flow diagram of FIG. 8A, at step 182 the qualitymetric measured at step 180 is compared to the previous best measuredQUALITY METRIC and, if better, then at step 184 the current qualitymetric is saved along with the global write current-setting controlsignal GLOBAL_CUR and local write current-setting control signalLOCAL_CUR. At step 186 the local write current-setting control signalLOCAL_CUR is incremented and a new quality metric is measured. Thisprocedure is repeated until the last local write current-setting controlsignal has been tried at step 188, wherein at step 190 the global writecurrent-setting control signal GLOBAL_CUR is incremented and the localwrite current-setting control signal LOCAL_CUR is reset to a minimum.Note that if the quality metric is the overwrite measurement describedabove, the loop will terminate as soon as the minimum write current (66₀-66 _(N)) is found that generates the desired overwrite measurementrather than test all combinations of global and local current-settingcontrol signals. After the last global current-setting control signalGLOBAL_CUR has been tested at step 192 (or the loop terminates early),then the flow diagram of FIG. 8B is executed.

At step 194 of FIG. 8B, the optimal global write current-setting controlsignal GLOBAL_CUR and local write current-setting control signalLOCAL_CUR saved at step 184 of FIG. 8A are stored in memory. If at step196 the last zone has not been reached, then the ZONE variable isincremented at step 198 and the flow diagram of FIG. 8A is re-executedfor the next zone on the current recording surface. If at step 200 thelast head has not been calibrated, then at step 202 the HEAD variable isincremented, the ZONE variable is reset to zero, and the calibrationprocedure 800 is re-executed for the next head. After the last head hasbeen calibrated, then at step 204 the optimal write current (66 ₀-66_(N)) for each head and in each zone are computed for the global writecurrent-setting control signal and the local write current-settingcontrol signals from the values stored in memory at step 194. Forexample, the global write current-setting control signal may be set tothe minimum write current (66 ₀-66 _(N)) calibrated for the disk array,and the local write current-setting control signals for the remainingheads set to an incremental offset added to the global writecurrent-setting control signal.

After the global and local write current-setting control signals havebeen computed, the bank servo write operation is performed tosimultaneously write the embedded servo sectors to all of the recordingsurfaces. At step 206 the ZONE variable is reset to zero to begin bankwriting the recording surfaces at the first zone. Then at step 208 theglobal and local write current-setting control signals for each head areloaded into the n-current preamplifier 64 and at step 210 the bank servowrite operation writes the embedded servo sectors to the tracks in thecurrent zone on all the recording surfaces. The ZONE variable is thenincremented at step 212, the global and local write current-settingcontrol signals for the next zone are loaded into the n-currentpreamplifier 64 at step 208, and the bank servo write operation at step210 writes the embedded servo sectors to the tracks in the next zone onall of the recording surfaces. This process is reiterated until the lastzone on all the disks has been servo written (i.e., the last ZONE isreached at step 214 in FIG. 8B) wherein the bank servo write procedureterminates.

We claim:
 1. A method of calibrating a write current-setting for writingservo sectors onto a recording surface through a head in a head diskassembly of a disk drive, the method comprising the steps of: (a)providing a preamplifier circuit having an input for receiving aselected control signal set by a current-setting value, the preamplifiercircuit causing current to flow through the head with a currentmagnitude determined by the current-setting value; (b) performing amultiple-pass process in which a series of current-setting values areset for the control signal for generating a plurality of quality metricseach indicative of a quality of the selected control signal, each passin the multiple-pass process including the steps of: providing a datasequence to the preamplifier circuit to cause a test pattern to bewritten to the recording surface; and reading the test pattern, andgenerating and storing at least one of the plurality of quality metrics;(c) evaluating the generated and stored quality metrics to select acurrent-setting value for the selected control signal; (d) setting theselected current-setting value selected in step (c) for the head; and(e) writing the servo sectors onto the recording surface through thehead; wherein the data sequence comprises a first data sequence and asecond data sequence and the test pattern comprises a first test patternand a second test pattern, the multi-pass process further comprises thesteps of: (f) providing the first data sequence to the preamplifiercircuit to cause the first test pattern to be written to the recordingsurface; (g) reading the first test pattern to generate a first readsignal having a low frequency signal component that depends on data inthe first data sequence; (h) filtering the first read signal to extractthe low frequency signal component, and measuring an energy in the lowfrequency signal component; (i) storing in memory the measured energy inthe low frequency signal component; (j) providing the second datasequence to the preamplifier circuit to cause the second test pattern tobe written over at least part of the first test pattern on the recordingsurface; (k) reading the second test pattern to generate a second readsignal; (l) filtering the second read signal to extract a residual lowfrequency signal component representing a residual of the low frequencysignal component; (m) measuring an energy in the residual low frequencysignal component; and (n) generating the quality metric as a ratio ofthe measured energy in the low frequency signal component to themeasured energy in the residual low frequency signal component.
 2. Themethod of calibrating a write current-setting as recited in claim 1,wherein: (a) the recording surface comprises a plurality of zones; and(b) the multi-pass process is performed for each zone of the recordingsurface to generate a current-setting value for each zone of therecording surface.
 3. A method of calibrating a plurality of writecurrent-settings for simultaneously writing servo sectors onto aplurality of recording surfaces through a plurality of heads in a headdisk assembly of a disk drive, the method comprising the steps of: (a)providing a preamplifier circuit having an input for receiving aselected control signal set by a current-setting value, the preamplifiercircuit causing current to flow through a selected one of the heads witha current magnitude determined by the current-setting value; (b)positioning the selected one of the heads over a respective one of therecording surfaces; (c) performing a multiple-pass process in which aseries of current-setting values are set for the control signal forgenerating a plurality of quality metrics each indicative of a qualityof the selected control signal, each pass in the multiple-pass processincluding the steps of: providing a data sequence to the preamplifiercircuit to cause a test pattern to be written to the recording surface;and reading the test pattern, and generating and storing at least one ofthe plurality of quality metrics; (d) evaluating the generated andstored quality metrics to select a current-setting value for theselected control signal; (e) repeating steps (b) through (d) for anotherone of the heads; (f) setting the selected current-setting valueselected in step (d) for each of the heads; and (g) simultaneouslywriting the servo sectors onto the plurality of recording surfacesthrough the plurality of heads; wherein the data sequence comprises afirst data sequence and a second data sequence and the test patterncomprises a first test pattern and a second test pattern, the multi-passprocess further comprises the steps of: (h) providing the first datasequence to the preamplifier circuit to cause the first test pattern tobe written to the recording surface; (i) reading the first test patternto generate a first read signal having a low frequency signal componentthat depends on data in the first data sequence; (j) filtering the firstread signal to extract the low frequency signal component, and measuringan energy in the low frequency signal component; (k) storing in memorythe measured energy in the low frequency signal component; (l) providingthe second data sequence to the preamplifier circuit to cause the secondtest pattern to be written over at least part of the first test patternon the recording surface; (m) reading the second test pattern togenerate a second read signal; (n) filtering the second read signal toextract a residual low frequency signal component representing aresidual of the low frequency signal component; (o) measuring an energyin the residual low frequency signal component; and (p) generating thequality metric as a ratio of the measured energy in the low frequencysignal component to the measured energy in the residual low frequencysignal component.
 4. The method of calibrating a plurality of writecurrent-settings as recited in claim 3, wherein: (a) each of therecording surfaces comprises a plurality of zones; and (b) themulti-pass process is performed for each zone of each recording surfaceto generate a current-setting value for each zone of each recordingsurface.